Modern wound-rotor synchronous machines typically require a stationary rotor field to interact with the stator field and produce torque at the machine shaft. The power to produce this stationary field is supplied from outside the motor in the form of DC current. Since the rotor of the machine rotates, it is necessary to supply power to the rotor through a rotating interface. Typically, this rotating interface is achieved through the use of brushes (stationary side) and slip rings (rotating side). This approach can be unsatisfactory with respect to long term durability (e.g., wear-out of brushes) and reliability (degradation of brush-to-slip-ring electrical contact in adverse environments).
Another approach, seen primarily in the power generation industry for large generators, is the use of a low frequency rotating transformer. The primary winding of the transformer is connected to the power grid through a rheostat or an autotransformer in order to adjust the input power. The secondary winding of the transformer rotates together with the rotor of the synchronous generator. A solid state or mechanical rectifier converts the AC power from the transformer secondary into DC power to be supplied to the field winding of the generator. Since such transformers operate at a relatively low grid frequency (e.g., 60 Hz), such a devices tend to be prohibitively large and heavy.
Accordingly, there is a need for more compact and efficient transformer designs for use in wound-rotor synchronous machines. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.